Radial Elongation

Although PET/CT modality is widely employed for diagnostic purpose, CT data acquired either for fusion of PET and CT images or for photon attenuation correction are affected by several factors that cause artifacts in PET/CT images. The factors include patient positioning, metal objects, CT contrast agents, truncation artifacts, and respiratory movement, which are discussed below. 

A simple method of recovering the truncated data is to linearly extrapolate the truncated CT projections to the extent of PET FOV and then setting the values to zero at the edge of the PET FOV. The attenuation correction by this method is adequate to remove the truncation artifacts in PET/CT images (Fig. 3.15). Using the iterative method rather than the filtered back projection technique also has shown to help to eliminate these artifacts. 

In recent years, 4D imaging involving gating of the respiratory cycle, similar to the cardiac-gated technique, has been introduced and a vast improvement 

A sinogram is a (a) hie containing individual annihilation events, (b) a matrix of parallel projection data, (c) a matrix of fan beam projection data, and (d) reconstructed image. 

In whole-body PET imaging, scanning is performed at 5-7 bed positions to scan the intended axial length of the body. At each bed position, there is an overlap of 3-5 cm between scans of two bed positions. Why  

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Inhibition of cell division, lack of spindle formation, inhibition of protein synthesis, increased cell size, radial elongation of cells, polyploid nuclei chromosome contraction, cell wall thickening. 

Radially-elongated parenehyma eells make up most rays. These eells resemble the axial parenchyma cells being reetangular in shape with more or less transverse endwalls. They have moderately thiekened walls perforated by small simple pits. 

The projection data acquired in the form of sinograms are affected by a number of factors, namely variations in detector efficiencies between detector pairs, random coincidences, scattered coincidences, photon attenuation, dead time, and radial elongation. Each of these factors contributes to the sinogram to a varying degree depending on the 2D or 3D acquisition and needs to be corrected for prior to image reconstruction. These factors and their correction methods are described below. 

Figure 3.11. An illustration of radial elongation. An off-center event (solid line) strikes the back of the detector pair tangentially. The X, Y positioning of the detectors (dash line) is a distance d away from the actual location of the positron annihilation, causing the blurring of the image. (Reprinted with the permission of The Cleveland Clinic Center for Medical Art Photography 2009. All Rights Reserved)...

Explain the error caused by radial elongation and how it can be corrected. 

By copolymerizing the monomers of two highly crystalline homopolymers it is possible to create a series of less crystalline copolymers with reduced levels of stiffness, elastic modulus and crystallinity depending on the blend level of the two monomers. This has been demonstrated by Soo H5nm Kim and coworkers at KIST, who have been able to synthesize a series of elastomeric (or rubbery) resorbable poly(L-lactide-co-e-caprolactone) copolymers. Because of their superior elastic behavior they have been used to prepare tubular scaffolds for vascular smooth muscle cell (SMC) culture under dynamic pulsatile conditions. We have melt spun and electrospun the 50/50 copolymer to fabricate a two-layer small-caliber vascular prosthesis prototype with radial elongation at break in excess of 300% (Fig. 15.33). " ... 

If the scatterers are elongated along the fibre direction and the two phases and their interfaces have the same consistency in both radial and z directions, a different expression of the Porod law can be derived. 

With disk diameters above 5.25 in., all parameters, eg, water absorption and thermal expansion, become more critical which aggravates the expansion or warp of disks. If in the future disk rotation speeds have to be increased significantly to boost data transfer rates, higher demands will be placed on warp (tilt angle) and modulus to avoid creeping (ie, irreversible elongation in radial direction). A survey of the requirement profile for the substrate material of optical disks is given in Table 5 (182,186,187,189). 

Delete the radial convection term but otherwise run the full simulation. This gives avgC = 0.5197. Now add the radial term to get 0.5347. The change is in the correct direction since velocity profile elongation hurts conversion. 

FIGURE 31.13 (a) Plot showing the stress-strain behavior of various irradiated rubbers, (b) Plot showing the variation of tensile strength and modulus of rubbers irradiated with different doses, (c) Plot showing the variation of hysteresis loss, set, and elongation at break of irradiated fluorocarbon rubbers. (From Banik, I. and Bhowmick, A.K., Radial. Phys. Chem., 54, 135, 1999. With permission.)... 

If the position of sample application and the point of entry of the mobile phase are at the center of the plate and the flow of mobile phase is towards the periphery of the plate, then this node of development is called circular chromatography [6,110]. Samples can be injected into the mobile phase, in which case they will be separated as a series of concentric rings. If the samples are applied as a cluster of spots in a radial pattern around the solvent entry position, after development, spots near the origin remain symmetrical and compact trtiile those near the solvent front are compressed in the direction of development and elongated at right angles to this direction. Figure 7.10(A). 

In the model, the internal structure of the root is described as three concentric cylinders corresponding to the central stele, the cortex and the wall layers. Diffu-sivities and respiration rates differ in the different tissues. The model allows for the axial diffusion of O2 through the cortical gas spaces, radial diffusion into the root tissues, and simultaneous consumption in respiration and loss to the soil. A steady state is assumed, in which the flux of O2 across the root base equals the net consumption in root respiration and loss to the soil. This is realistic because root elongation is in general slow compared with gas transport. The basic equation is... 

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